1. Field of the Invention
The present invention relates to an improvement of a fluorescent composition. More particularly, it relates to a fluorescent composition capable of emitting light of high luminance under low-velocity electron excitation at an acceleration voltage of not higher than a few hundreds volts and having the decrease in luminance with time during the use for a long time reduced.
2. Description of Background
Zinc activated zinc oxide phosphors (ZnO:Zn) are well known as phosphors which can emit light of high luminance under excitation by low-velocity electron beams at an acceleration voltage of not higher than 1 kV, particularly not higher than a few hundreds volts. When excited by low-velocity electron beams, the ZnO:Zn phosphors emit green-white light of high luminance, and the fluorescent display devices having fluorescent screens composed of such ZnO:Zn phosphors are widely used as display elements for table-top calculators, various meters for vehicles and electrical appliances such as VTR.
In recent years, reflecting the expansion of the field of the application of the fluorescent display devices, a variety of emitting colors are desired for the fluorescent display devices. Various developments have been made on the fluorescent materials which are capable of emitting light other than green light under low-velocity electron excitation. As a result, some fluorescent materials have been developed. Among them, fluorescent compositions are known which are prepared by mixing a conductive material such as indium oxide (In.sub.2 O.sub.3), zinc oxide (ZnO) or tin oxide (SnO.sub.2) to a sulfide phosphor consisting essentially of a sulfide of the formula (Zn.sub.1-x, Cd.sub.x)S wherein x is a number satisfying a condition of 0.ltoreq.x.ltoreq.1, as host material, which is activated by zinc (Zn), silver (Ag), gold (Au), copper (Cu) or manganese (Mn) and, if necessary, co-activated by halogen, aluminum (Al), gallium (Ga) or indium (In) (Japanese Examined Patent Publications No. 33153/1984 and No. 33155/1984 and Japanese Examined Patent Publication Nos. 44035/1987 and 53554/1987). (A phosphor consisting essentially of a sulfide of the formula (Zn.sub.1-x, Cd.sub.x)S wherein x is a number satisfying a condition of 0.ltoreq.x.ltoreq.1, as host material, will generally be referred to hereinafter as "a (Zn.sub.1-x, Cd.sub.x)S type sulfide phosphor".) When excited by low-velocity electron beams at an acceleration voltage of not higher than 1 kV, these fluorescent compositions exhibit emission of visible lights of high luminance ranging from blue to red depending upon the solid solution ratio (x value) of Cadmium (Cd) to Zinc (Zn) in the (Zn.sub.1-x, Cd.sub.x)S type sulfide phosphor host material as one of the constituting components of the fluorescent compositions or upon the combination of activators and co-activators. Therefore, by using such fluorescent compositions as fluorescent screens, it has been made possible to obtain a variety of colors for fluorescent display devices, and the amount of information has been remarkably improved. However, fluorescent display devices using, as fluorescent screens, fluorescent compositions composed mainly of such (Zn.sub.1-x, Cd.sub.x)S type sulfide phosphors, have a serious drawback that when operated for a long period of time, there will likely be a decrease in luminance of the fluorescent layer or non-uniformity in emission, which is considered to be attributable to a deterioration of the (Zn.sub.1-x, Cd.sub.x)S type sulfide phosphor in the fluorescent screen, or deposition of a decomposed product of the phosphor to the filament. Therefore, it has been desired to develop a fluorescent composition which hardly undergoes a deterioration or decomposition even under excitation by low-velocity electron beams for a long period of time and which scarcely brings about a decrease in luminance or non-uniformity in emission.